In certain means of transportation, especially aircraft, provision is often made for a temperature-monitoring means to check whether there is any abnormal heating at determined locations (in particular the engines) which are most at risk for fire. To this end, use is made of a coaxial wire or cable which has, between the central conductor (or core) and the outer conductor, a sheathing made of a material (which is generally semi-conducting), with resistivity varying with temperature. This cable generally follows a path that passes through the zones subject to the greatest risk, and the monitoring is done by continuously or periodically measuring the resistance at one end of the cable between the core and the outer conductor, the said resistance depending on the resistivity of the material which comprises the intermediate element and, hence, on the temperature. If the relative variation of the resistance exceeds a pre-determined value, an alarm is triggered.
The resistance measured between the core and the outer conductor is inversely proportional to the length of the cable. Thus, it follows that measuring resistance alone does not give a precise determination of the variation in temperature since, for one and the same temperature, the variation in resistance depends on the length of that part of the cable on which the heating occurs. This is why, until now, a fire wire monitoring device has been used only to trigger one alarm. But even for this method of single use, the lack of precision may cause an alarm to be triggered when the heating is still at a permissible level.